1. Field of the Invention
This invention relates to a lockup device of a torque converter. More specifically, the present invention relates to a lockup device having a plurality of coil springs aligned in a circumferential direction.
2. Background Information
Torque converters usually include a fluid coupling mechanism for transmitting torque between the crankshaft of an engine and the input shaft of an automatic transmission. A torque converter has three types of runners (impeller, turbine, stator) located inside for transmitting the torque by means of an internal hydraulic oil or fluid. The impeller is fixedly coupled to the front cover that receives the input torque from the crankshaft of an engine. The hydraulic chamber formed by the impeller shell and the front cover is filled with hydraulic oil. The turbine is disposed opposite the front cover in the hydraulic chamber. When the front cover and the impeller rotate together, the hydraulic oil flows from the impeller to the turbine, and the turbine rotates. As a result, the torque is transmitted from the turbine to the main drive shaft of the transmission.
Generally, a torque converter can perform smooth acceleration and deceleration because it transmits a power via fluid. However, an energy loss occurs due to slip of the fluid, resulting in low fuel consumption. Accordingly, in recent years to improve fuel efficiency, some of the conventional torque converters have included a lockup device for mechanically coupling a front cover on an input side and a turbine on an output side. Specifically, the lockup device is disposed in a space located axially between the front cover and the turbine. When the torque converter reaches predetermined operating conditions, the lockup device of the torque converter causes power from the crankshaft of the engine to be directly transmitted to the automatic transmission, and thus, bypassing the fluid coupling device.
Usually, such lockup devices typically include a disk-like piston and a damper mechanism having a retaining plate, torsion springs and a driven member. The piston can be pressed against the front cover. The retaining plate is secured to an outer peripheral section of the piston. The torsion springs are supported by the retaining plate in a rotational direction and at the outer peripheral side of the retaining plate. The driven member supports the opposing ends of each torsion spring in a rotational direction. The driven member is secured to a turbine shell or a turbine hub of the turbine.
As the lockup device is activated, torque is transmitted from the front cover to the piston and then to the turbine through the torsion springs. Furthermore, as the torque fluctuations are transmitted from an engine to the lockup device, the torsion springs are compressed between the retaining plate and the driven member in the damper mechanism, such that torsional vibrations are absorbed and dampened. In other words, the damper mechanism unctions as a torsional vibration dampening mechanism to dampen vibration in the lockup clutch.
The piston carries an annular friction member adhered to a position opposed to a flat friction surface of the front cover. This portion of the piston and the friction surface of the front cover form a clutch coupling portion-of the lockup device. When a clutch coupling portion of the lockup device operates, the torque is transmitted from the front cover to the piston. The torque thus transmitted is further transmitted from the retaining plate to the driven plate via the coil springs, and then to the turbine. Torsional vibrations transmitted from the front cover are absorbed and dampened by the coil springs that are compressed between the retaining plate and the driven plate.
The piston is disposed to divide the space between the front cover and the turbine into a first hydraulic chamber on the front cover side and a second hydraulic chamber on the turbine side. As a result, the piston can move axially close to and away from the front cover due to the pressure difference between the first hydraulic chamber and the second hydraulic chamber. When the hydraulic oil in the first hydraulic chamber is drained and the hydraulic pressure in the second hydraulic chamber increases in pressure, the piston moves toward the front cover side. This movement of the piston causes the piston to strongly press against the front cover.
In the conventional lockup device, the operation of the piston is controlled by the working fluid flowing through the main unit of the torque converter. More specifically, a hydraulic operation mechanism in an external position supplies the working fluid to a space between the piston and the front cover when the lockup device is disengaged. This working fluid flows radially outward through the space between the front cover and the piston, and then flows from its radially outer portion into the main unit of the torque converter. When the lockup device is engaged, the working fluid in the space between the front cover and the piston is drained from its radially inner portion so that the piston moves toward the front cover. Thereby, the friction member arranged on the piston is pressed against the friction surface of the front cover. In this manner, the torque of the front cover is transmitted to the turbine via the lockup device.
There is an increasing demand for higher performance damper mechanisms. The demand dictates for damper mechanisms that can be utilized at lower vehicle speeds and higher torque levels. In a recently introduced torque converter, torque is transmitted through fluid only as acceleration commences from a standstill. In other words, the torque transmitted via the fluid is performed only during a start operation. In such torque converter, the lockup device is set to an engagement state as soon as the vehicle reaches a certain low speed such as 10 km/h. In such a vehicle that has an increased lockup range, the performance of the coil springs needs to be improved such that the torsional vibrations due to torque variations of the engine are absorbed and dampened sufficiently.
However, in a conventional lockup device described above, a coil diameter of the coil spring cannot be sufficiently increased due to the following reason. Radially opposite sides of the coil springs are supported by the retaining plate. The retaining plate has at its outer periphery a cylindrical portion for bearing loads from the coil springs. In other words, the coil springs are arranged on the outer peripheral portion of the retaining plate and tend to move radially outward due to a centrifugal force that causes outward deflection of the coil springs. The coil diameters of the coil springs cannot be increased sufficiently because the member that radially supports opposite sides of the coil springs further limits the space available to the coil springs. Furthermore, the lockup device is arranged in an axially restricted space within the torque converter.
In view of the above, there exists a need for a lockup device of a torque converter which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.